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six other planets, and eighteen moons belonging to them.
Charles. And you have not reckoned the four newly discovered little planets, which Dr. Herschel denominates Asteroids, but which are known by the name of Ceres Ferdinandea, Pallas, Juno, and Vesta.
Tutor. Well then; the sun to these is the perpetual source of light, heat, and motion; and to more distant worlds it is a fixed star, and will appear to some as large as Arcturus, to others no larger than a star of the sixth magnitude, and to others it must be invisible, unless the inhabitants have the assistance of glasses, or are endowed with better eyes than ourselves.
James. Pray, Sir, how swift do you reckon that the particles of light move?
Tutor. This you will easily calculate, when you know that they are only about eight minutes in coming from the sun.
Charles. And if you reckon the sun to be at the distance of ninety-five millions of miles from the earth ; light proceeds at the rate, nearly, of twelve millions of miles in a
minute, or at 200,000 miles in a second of time. But how do you know that it travels so fast?
Tutori It was discovered by M. Roemer, who observed that the eclipses of Jupiter's satellites took place about sixteen minutes later, if the earth were in that part
of its orbit, which is farthest from Jupiter, than if it were in the opposite point of the heavens.
Charles I understand this : the earth may sometimes be in a line between the sun and Jupiter, and at other times the sun is between the earth and Jupiter; and therefore, in the latter case, the distance of Jupiter from the earth, is greater than in the former, by the whole length of its orbit.
Tutor. In this situation, the eclipse of any of the satellities is, by calculation, sixteen minutes later than it would be, if the earth were between Jupiter and the sun ; that is, the light flowing from Jupiter's satellites is about sixteen minutes in travelling the length of the earth's orbit, or 190 millions of miles.
Fames. It would be curious to calculate how much faster light travels than a cannon-ball.
Tutor. Suppose a cannon-ball to travel at the rate of twelve miles a minute ; and light to move a million of times faster than that; yet Dr. Akenside conjectures that there
may be stars so distant from us that the light proceeding from them has not yet reached the earth :
-Whose unfading light Has travell’d the profound six thousand years, Nor yet arriv’d in sight of mortal things.
Charles. Was it to this author that Dr. Young alludes in these lines?
How distant some of the nocturnal suns !
Tutor. He probably referred to Huygens, an eminent, astronomer, who thew out the idea before Akenside was born.
. In no other way: and there
James. And you say the particles of light move in all directions.
Tutor. Here is a sheet of thick brown paper, I make only a small pin-hole in it, and then through that hole, I can see all the objects, such as the sky, trees, houses, &c. as I could if the paper were not there,
Charles. Do we only see objects by means of the rays of light which flow from them?
Tutor. I In no other way: and therefore the light that comes from the landscape, which I view by looking through the small hole in the paper, must come in all directions at the same time.Take another instance; if a candle be placed on an eminence in a dark night, it may be seen all round for the space of half a mile : in other words, there is no place within a sphere of a mile in diameter, where the candle cannot be seen, that is, where some of the rays
from the small flame will not be found.
James. Why do you limit the distance to half a mile?
Tutor. The distance, of course, will be greater or less, according to the size of the candle: but the degree of light; like heat, diminishes in proportion as you go
farther from the luminous body.
Charles. Does it follow the same law as gravity?*
Tutor. It does : the intensity or degree of light decreases as the square of the distance from the luminous body increases.
James. Do you mean, that at the distance of two yards from a candle, we shall have four times less light, than we should have, if we were only one yard from it?
Tutor. I do: and at three yards distance, nine times less light ; and at four yards' distance you will have sixteen times less light than you would were you within a yard of the object.--I have one more thing to tell you: light always moves in straight lines.
James. How is that known?
* See Scientific Dialogues, Vol. I. Conversation VII.